1. Field of the Invention
The present invention relates to an output stage circuit for gate driving circuit in an LCD, and more particularly, to an output stage circuit for gate driving circuit in an LCD for reducing the feed-through effect phenomenon.
2. Description of the Prior Art
In liquid crystal displays (LCDs), if a gate driving signal outputted from the gate driving circuit falls too fast, which means its falling edge is too sharp, the Gamma data stored therein become incorrect, because the effect of feed-through phenomenon thought parasitic capacitance. More specifically, if the voltage of the gate driving signal drops too fast, the signal will be coupling to the thin film transistors of the pixels corresponding to the gate line through the intrinsic capacitors of the thin film transistors, causing the final voltage on the liquid crystal particle differs from the voltage the source driving circuit writes. Such phenomenon is called feed-through phenomenon.
FIG. 1 shows a conventional modulation mechanism to solve the feed-through phenomenon. In the LCD 100, a power circuit 120 provides a fixed voltage VO, a timing controller 140 controls a gate driving circuit 130 and a source driving circuit 150. To solve the feed-through phenomenon, a modulation circuit 110 is added in the LCD 100 to modulate the waveform of the output voltage VO from the power circuit 120 to be modulated VM. Then the modulated voltage VM is provided to the gate driving circuit 130 as its supply voltage. The modulation circuit 110 is also controlled by the timing controller 140. More particularly, when the timing controller 140 controls the gate driving circuit 130 to lower the gate driving signal SG, the modulation circuit 110 is also controlled to modulate the output voltage VO lowered as shown in FIG. 1 (to become the modulated voltage VM). Consequently, the final voltage supplied to the gate driving circuit 130 drops when the gate driving circuit 130 lowers the gate driving signal SG. In this way, the ability of the gate driving circuit 130 is decreased, and thus the slope of the gate driving signal SG becomes more moderate.
FIG. 2 shows the waveform of the gate driving signal SG before/after the output voltage VO is modulated. FIG. 2A shows the output voltage VO is not modulated, causing the gate driving signal SG falls sharply. FIG. 2B shows the output voltage VO is modulated to be the modulated voltage VM and then is supplied to the gate driving circuit 130. In FIG. 2B, the gate driving signal SG falls moderately by the drop of the supplied voltage (VM).
However, to solve the feed-through phenomenon, an LCD has to be added with the modulation circuit, causing wasting on total power consumption and cost of the LCD, which is inconvenient for users.